diff --git a/src/objective-c/ProtoRPC/ProtoRPC.m b/src/objective-c/ProtoRPC/ProtoRPC.m
index 517c4812a783424faea565e32687bcbe925089e5..4da646d7b453e1ab300c594e1d74b0a84ed6bfec 100644
--- a/src/objective-c/ProtoRPC/ProtoRPC.m
+++ b/src/objective-c/ProtoRPC/ProtoRPC.m
@@ -66,6 +66,8 @@
   // A writer that serializes the proto messages to send.
   id<GRXWriter> bytesWriter =
       [[[GRXWriter alloc] initWithWriter:requestsWriter] map:^id(GPBMessage *proto) {
+        // TODO(jcanizales): Fail with an understandable error message if the requestsWriter isn't
+        // sending GPBMessages.
         return [proto data];
       }];
   if ((self = [super initWithHost:host method:method requestsWriter:bytesWriter])) {